Save value on first occurrence in Animation Nodes
Attributes can be stored in memory during the life time of the blender instance as described in this answer. In this particular case, the logic is as follows. We have two variables occurred, storedValue:


If the current frame is equal to the start frame, reset storedValue to some initial value and reset occurred to False.
If trigger value is positive:


If occured is False:


Set storedValue to the value.

Set occurred to True.